1. Field of the Invention
The present invention relates to a conductive resin composition from which a molded product can be formed wherein the material has high conductivity, excellent effect of shield against electromagnetic waves, high rigidity, and stable physical properties as well as excellent surface smoothness. More particularly, the present invention is concerned with a conductive resin composition which can be advantageously used as a housing part for electrical apparatus, especially as a housing for personal computer and a housing for information terminal such as a mobile phone, and processes for preparing the same.
2. Prior Art
Resin compositions used in electromagnetic wave shield materials, such as a housing and a cover for personal computer, are required to have such high conductivity that they can shield the contents from electromagnetic waves. As a resin composition which meets such a demand, a resin composition comprising a thermoplastic resin having conductive filler dispersed therein as a matrix has been proposed.
As filler incorporated into the thermoplastic resin, carbon fiber having conductivity and an effect as a reinforcing agent has generally been used. A molded product comprising a resin composition in which carbon fiber is formulated into a thermoplastic resin has excellent rigidity and excellent conductivity. However, when such a molded product is used as a shield against electromagnetic waves, it must have even higher conductivity, and, for obtaining the molded product having a higher conductivity, it is necessary to incorporate a large amount of carbon fiber into the thermoplastic resin.
However, when a molded product is formed from the resin composition having incorporated thereinto a large amount of carbon fiber as mentioned above, the material has problems not only in that the molded product suffers large warpage due to orientation of the carbon fiber, but also in that the large amount of carbon fiber incorporated tends to cause the physical properties of the material to be lowered and further cause the cost to increase.
For solving the above problems, Japanese Provisional Patent Publication No. 181532/1991 has proposed a resin composition comprising a thermoplastic resin, such as a polyester resin, a polyethylene resin, a polypropylene resin, a polyamide resin, an acrylonitrile-butadiene-styrene (hereinafter referred to as “ABS”) copolymer, a polyacetal resin, a modified polyphenylene oxide resin, or a polycarbonate resin, which resin has incorporated thereinto 5 to 30% by weight of swollen graphite having a particle diameter of 20 μm or less. A molded product comprised of this resin composition suffers small warpage, but it has low conductivity, as compared to the molded product containing carbon fiber.
On the other hand, the conductivity is largely influenced by the carbon fiber length, and, when a thermoplastic resin and carbon fiber are kneaded together, the carbon fiber is broken into short one, so that a satisfactory conductivity cannot be obtained. For solving such a problem, long-length fiber pellets having a pellet size which is the same as the carbon fiber length (pellet size=carbon fiber length) are used. However, since all the fiber is long-length fiber or a large amount of long-length fiber is incorporated, there are problems that the surface of the molded product is roughened or the physical properties are unstable.